Direct conversion of sunlight into electric power by solar cells is one of the only clean energy resources. In solar cells electricity is produced without the exhaust of greenhouse gases and without the leftover of nuclear waste. Dye-sensitized solar cells (DSSC) appear to have significant potential as low cost alternatives to conventional p-n junction solar cells. Being transparent to most of the visible range of wavelengths makes the DSSC one of the only candidates for power windows that will further decrease the cost of this clean energy.
DSSCs consist of a nanocrystalline, mesoporous network of a wide bandgap semiconductor, which is covered with a monolayer of dye molecules. The semiconductor is deposited onto a transparent conductive oxide electrode, through which the cell is illuminated. The TiO2 pores are filled with a redox mediator, which acts as conductor, connected to a counter electrode. Upon illumination, electrons are injected from the photo-excited dye into the semiconductor and move towards the transparent conductive substrate, while the electrolyte reduces the oxidized dye and transports the positive charges to the counter electrode. Such systems can reach solar to electric conversion efficiencies of about 12% but are still not produced on a large scale.
Lowering the cost of solar cell production is one of the most important aims in photovoltaics. Since the report of O'Regan and Gratzel (1991), DSSCs have been developed very quickly as one of the most promising devices for solar energy conversion. Nanocrystalline titanium dioxide porous film is the essential part of DSSC. The conventional method of preparation of porous nanocrystalline TiO2 electrodes is based on viscous TiO2 paste with organic additives that can be deposited on a substrate by screen-printing or blade coating. Organic additives enable the preparation of crack-free thick films (10-18 μm) in one deposition. Thermal treatment at 450-550° C. is used to remove organics and sinter the nanoparticles together to establish mechanical stability and electrical connection in the network.
Low temperature preparation of nanocrystalline titania (TiO2) films is necessary for the fabrication of light weight, flexible, thin and low cost DSSCs on plastic substrates. Various methods have been employed to prepare titania mesoporous films at temperatures below 150° C. These temperatures permit the use of plastic substrates such as polyethylene terephthalate (PET). Among the reported methods, the most efficient ones are hydrothermal crystallization (Oekermann et al., 2004; Zhang et al., 2003), mechanical pressing technique (Lindström et al., 2002; Boschloo et al., 2002; Lindström et al., 2001a; Lindström et al, 2001b) and electrophoretic deposition (EPD) assisted by chemical vapor deposition (CVD) and UV treatment (Miyasaka et al., 2002; Murakami et al., 2003). However, cell efficiencies are still low in comparison with the best cells prepared on the basis of sintered films at 450-550° C. (10.4%) (Nazeeruddin et al., 1993; Nazeeruddin et al., 2001).
Electrophoretic deposition (EPD) is a useful way for the preparation of thick binder-free particulate films on conductive substrates in much shorter time compared to the other coating techniques. EPD is especially attractive because of the low cost, simple setup, formation of uniform layers of controlled thickness and homogeneous microstructure (Sarkar and Nicholson, 1996; Zhitomirsky, 2002; Boccaccini and Zhitomirsky, 2002). So far, studies on EPD of TiO2 nanoporous films for DSSCs are very sparse.
Miyasaka et al (2002) and Murakami et al (2003) reported the EPD of the commercially available TiO2 nanoparticle powders, P-25 and F-5, from dry mixed solvent of tert-butanol and acetonitrile. Direct current (DC) electric field of 200 V/cm was applied for 0.5 to 1 min. As a result, TiO2 nanoporous films with thickness of 7-13 μm were prepared and after drying at 90° C. used for fabrication of DSSCs. These DSSCs yielded solar-to-electric conversion efficiency of 2.0% when illuminated by 100 mW/cm2 white light. It is worth noting that conductive glass was used as electrode substrate in this case. After post-treatment of the electrodeposited film by chemical vapor deposition (CVD) of Ti(OC3H7)4, followed by exposure to UV-light, conversion efficiency became 3.7%. After post-treatment by microwave irradiation, the conversion efficiency reached 4.1%. With respect to commercial production, the process used by Miyasaka et al and Murakami et al suffers from several shortcomings. First, cell efficiencies are still low, even with post-treatments. Second, acetonitrile is a highly toxic and carcinogenic chemical, and cannot be used without special precautions. Finally, the post-treatment processes, CVD and UV, are too expensive to scale up.
Much lower light to electricity conversion efficiencies have been reported by other authors for DSSCs prepared by EPD (Matthews et al., 1994; Fujimura and Yoshikado, 2003). Fujimura and Yoshikado (2003) used ion-free water without binder as solvent for EPD, which has the advantages of low cost, industrial safety, environmental protection and the minimization of contamination in produced layers. However, the main problem associated with the applying of water-based suspensions for EPD is the formation of gas by the hydrolysis of water above a DC voltage of about 1.4 V, resulting in large pinholes in deposited layers, lack of film uniformity and adherence (Zhitomirsky, 2002; Tabellion and Clasen, 2004). Fujimura and Yoshikado (2003) worked at DC voltages higher than 1.5 V, with relatively low deposition rates, e.g., a 15 μm thick film required a deposition duration of 530 sec (about 9 min). The DSSCs fabricated from these electrodes showed low open circuit voltage (Voc) (maximum value 480 mV) and low short circuit current density (Jsc) (maximum value 0.4 mA/cm2). No information on the fill factor (FF) and efficiency of these cells was provided.